Clarify and fix docstrings

skimage/segmentation/boundaries.py

@@ -203,6 +203,10 @@ def mark_boundaries(image, label_img, color=(1, 1, 0),
     if marked.ndim == 2:
         marked = gray2rgb(marked)
     if mode == 'subpixel':
+        # Here, we want to interpose an extra line of pixels between
+        # each original line — except for the last axis which holds
+        # the RGB information. ``nd.zoom`` then performs the (cubic)
+        # interpolation, filling in the values of the interposed pixels
         marked = nd.zoom(marked, [2 - 1/s for s in marked.shape[:-1]] + [1],
                          mode='reflect')
     boundaries = find_boundaries(label_img, mode=mode,